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United States Patent | 6,157,558 |
Wong | December 5, 2000 |
An SRAM-based CAM cell and CAM array architecture reduce transistor count and memory size by replacing pass transistors and search transistors of conventional SRAM-base CAM cells with a pair of transistors having gates coupled to bit lines. The two bit-line-controlled transistors in a CAM cell are between storage nodes and a word/match line for the CAM cell. The sizes of pull-up and pull-down devices in the CAM cells are selected so that grounding a storage node to a word/match line through one of the two bit-line-controlled transistors can change the bit stored in a CAM cell, but applying a voltage (near the supply voltage) from the word/match line through either of the two bit-line-controlled transistors to a storage node cannot change the bit or data stored in a CAM cell. Accordingly, a write operation grounds a selected word/match line and applies a voltage to the unselected word/match lines. A search operation charges all word/match lines and senses the word/match lines. Addition of a mask element that controls the connection of the CAM cell to the word/match line can convert a binary CAM architecture to a ternary CAM architecture. The mask element optionally includes circuitry that causes the mask element to power up in a known state. Within the ternary CAM cell, a bypass transistor can be provide to bypass the effect of the mask element and facilitate write operations or temporarily suspend local masking.
Inventors: | Wong; Sau-Ching (Hillsborough, CA) |
Assignee: | SanDisk Corporation (Campbell, CA) |
Appl. No.: | 316499 |
Filed: | May 21, 1999 |
Current U.S. Class: | 365/49 |
Intern'l Class: | G11C 015/00 |
Field of Search: | 365/49,154,156 |
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TABLE 1 ______________________________________ CAM Cell Voltages during a Write Bit to BL 161 BL 162 Resulting Resulting Store Voltage Voltage N1 State N2 State ______________________________________ 0 0 Vcc 0 Vcc 1 Vcc 0 Vcc 0 ______________________________________
TABLE 2 ______________________________________ CAM Cell Voltages during a Search Stored Search BL BL Tms Tms Bit N1 N2 Bit 162 161 332 331 Result ______________________________________ 0 0 Vcc 0 0 Vcc NC NC Match 0 0 Vcc 1 Vcc 0 C NC No Match 1 Vcc 0 0 0 Vcc NC C No Match 1 Vcc 0 1 Vcc 0 NC NC Match ______________________________________